The invention relates to a process for measuring particle size by measuring the attenuation of radiation after its passage through a defined measuring section containing a disperse system.
Many fields of process engineering require an exact characterization of disperse systems. In disperse systems there is a disperse phase, i.e. solid, liquid or gaseous particles in a continuous phase, that is, a liquid or gaseous phase. Characterization is generally based on particle size distribution and particle volume concentration. Ideally these quantities are measured in situ so as to avoid errors due to sampling, dilution and the like. There is also a need for simple and cost-effective process control methods.
There are known particle size measurement processes which use statistical data taken from transmission measurements. For this purpose a measurement of mean transmission (dependent solely on the particle projection area concentration, or on volume concentration and mean particle size) is expressed in relation to the standard deviation of transmission measured with a fixed beam diameter. The standard deviation is dependent on the volume concentrationxe2x80x94although there is no single-valued function for this (Gregory (in the Journal of Colloid and Interface Science, Vol. 105, No. 2, 1985, p. 357) FIG. 11, "PHgr"=CV)xe2x80x94and on the mean particle size. By this means it is possible to determine the concentration and mean particle size for a specific range of values, but it is not possible to measure a particle size distribution in this way.
The objective of the invention is to provide an improved process for measuring particle size which eliminates the need to perform a calibration procedure using the material being measured.
According to the invention this objective is achieved on the basis of a generic process by the temporally fluctuating transmission signal is recorded with variable temporal or spatial resolution. The transmission signals undergo a non-linear operation and the result of that non-linear operation is represented and interpreted as a spectral curve, i.e. as a function of the spatial or temporal resolution.
According to the invention this takes advantage of the fact that the temporal fluctuations of the transmission signal, together with the electrical noise, are an expression of the probability of the presence of various particle size classes in the volume being measured. According to the invention described herein, the information on particle size distribution and particle volume concentration contained in the fluctuation of the transmission signal is used systematically for particle size measurement. An essential step in the process according to the invention is that the transmission signals undergo a non-linear operation. The non-linear operation can, for example, be a squaring operation, logarithmisation, or indeed an analytical function, for example, a function of the form (T)N or exp {T}, where T is transmission and N any real number. However, in general terms it is also possible to use other non-linear operations in the process according to the invention.
It is advantageous if the signal is recorded with high temporal and spatial resolution and represented and analyzed as an auto power density spectrum in the form of a special non-linear operation.
Signal recording and processing with variable temporal resolution can be achieved by a variety of low-pass, high-pass and/or band-pass combinations, and the result of the non-linear operation can be represented and analyzed of the low pass, high-pass or band pass.
The signals can be recorded digitally, and a sliding average with variable averaging parameter can be formed as digital low pass, high pass and/or band pass.
According to an especially advantageous embodiment, the cross-section of the measurement beam can be variable in size and/or shape, so as to produce an optic low pass, high pass and/or band pass.
It is especially advantageous if the process consists of the combination of the following process steps:
creation of the signals in a small optic measurement cross-section,
variable electrical or digital averaging,
subjecting the average values formed to a non-linear operation,
repeating the previous process steps with at least one larger measurement cross-section, and
comparison of the results from the non-linear operations, and the determination on that basis of the particle size distribution, particle concentration and particle velocity.
Alternatively, an advantageous embodiment of the process consists of the combination of the following process steps:
creation of signals and variation of optic measurement cross-sections,
subjecting these signals to a non-linear operation,
creation of signals using a very small optic measurement cross-section,
known fixed or variable electronic and digital averaging of the signals produced,
subjecting these signals to the same non-linear operation as they have already undergone,
comparison of the results from the non-linear operations and determination of the particle size distribution, particle concentration and particle velocity on the basis of that comparison.
According to an alternative embodiment of the process according to the invention, the primary beam can be divided into several sub-beams and passed through the measurement section in the form of those sub-beams. These sub-beams can be used to form variable optic low passes or band passes, by additive or subtractive superposition of the transactions measured with the sub-beams.
The creation of such sub-beams makes it possible to carry out the non-linear operation by means of a non-linear combinationxe2x80x94a multiplication, for examplexe2x80x94of the transmissions of two or more sub-beams. According to various embodiments of the invention the sub-beams can run through the suspension in different directions. First, they can run parallel to a plane set transverse to the direction of flow. Alternatively, they can run parallel to a plane which is set parallel to the direction of flow of the suspension. Finally, the sub-beams can intersect at a single point, i.e. the measurement volume. Alternatively they may cross in various different planes lying one above the other, viewed in the direction of flow of the suspension.
The process according to the invention is now described on the basis of a preferred embodiment example illustrated in the accompanying drawings, in which
FIG. 1 is a graph of measurement of a signal over time;
FIG. 2 is a graph of a curve of average value of the signal for mono-disperse glass beads over the beam diameter;
FIG. 3 is a graph of the average value of the signal of FIG. 2 in standardized form;
FIG. 4 is a graph of the average value of the signal over particular diameter for a mix of two mono-disperse fractions;
FIG. 5 schematically illustrates intersection of two measurement beams;
FIG. 6 is a graph of the log of the average signal value of the mono-disperse phase suspension of FIG. 5 against distance between the two beams of FIG. 5; and
FIG. 7 is a graph of the log of the cross-correlation product of the two beams of FIG. 5 for any fixed distance therebetween as a function of standardized placement in time.